The invention relates to a method for operating an internal combustion engine, especially of a motor vehicle, wherein fuel is injected directly into a combustion chamber in a first operating mode during an induction phase, in a second operating mode during a compression phase or in a third operating mode during the intake and compression phases and is ignited. Likewise, the invention relates to a corresponding control apparatus for an internal combustion engine, especially of a motor vehicle, as well as a corresponding engine, especially for a motor vehicle.
A method, control apparatus and internal combustion engine of this kind are known, for example, in a so-called gasoline direct injection. There, the fuel is injected into the combustion chamber of the engine in a homogeneous operation during the induction phase or in a stratified operation during the compression phase. The homogeneous operation is preferably provided for the full-load operation of the engine; whereas, the stratified operation is suitable for the idle and part-load operation. In a direct-injecting engine of this kind, for example, there is a switchover between the above-mentioned operating modes in dependence upon the requested torque.
In a double injection, fuel is injected into the combustion chamber during the induction phase and during the compression phase. A reduction of the knock sensitivity of the engine can be achieved with this combination of the homogeneous operation and the stratified operation.
A further reduction of the knock sensitivity can be achieved by a retardation of the ignition angle for the ignition of the spark plug. This, however, causes a collapse of torque which is the same as a deterioration of the efficiency.
The task of the invention is to provide a method for operating an internal combustion engine with which the knock sensitivity of the engine can be further reduced without significantly reducing the efficiency thereof.
This task is solved according to the invention with a method of the type initially mentioned herein in that the mass of the fuel, which is injected during the induction phase, is reduced in the third operating mode when there is a knocking of the engine. The task is correspondingly solved with a control apparatus and an internal combustion engine of the respective types mentioned initially herein.
The air/fuel mixture injected in homogeneous operation becomes leaner because of the reduction of the fuel mass injected during the induction phase. A homogeneous operation, which becomes leaner in this manner, affords the advantage that the knock sensitivity of the engine becomes less. The engine is thereby less sensitive to knocking. It is therefore not necessary to retard the ignition angle in order to avoid knocking of the engine. In this way, no torque is destroyed so that also no reduction of efficiency is present.
In an advantageous embodiment of the invention, the mass of the fuel, which is injected during the compression phase, is increased in the same manner as the mass of the fuel, which is injected during the induction phase, is reduced. In this way, overall, the injected fuel mass is held essentially constant. The torque, which is generated by the engine, thereby remains essentially the same and the knock sensitivity of the engine is, however, reduced as already mentioned.
In an advantageous embodiment of the invention, fuel is injected directly by the injection valve into the combustion chamber in the third operating mode after the ignition via the spark plug. In total, a triple injection is carried out. In this way, the fuel, which is to be injected, can be better distributed over the different individual injections.
In an advantageous embodiment of the invention, the mass of the fuel, which is injected after the ignition, is increased in the same manner as the mass of the fuel, which is injected during the induction phase, is reduced. In this way, overall, the injected fuel mass is held essentially constant. The fuel mass, which is injected during the compression phase, does not have to be significantly changed.
It is especially advantageous when the mass of the fuel, which is injected during the compression phase, is held to a value which is less than a pregivable or applicable limit. In this way, the fuel mass, which is injected during the compression phase, can be held under that value which ensures a combustion below the soot limit. At the same time, a reduction of the fuel mass, which is injected during the induction phase, can however be compensated by an increase of the fuel mass which is injected after the ignition. In total, essentially the same fuel mass is injected into the combustion chamber. However, on the one hand, the knock sensitivity of the engine is reduced because of a reduction of the fuel mass which is injected during the induction phase without, however, exceeding the soot limit of the fuel mass injected during the compression phase.
Of special significance is the realization of the method of the invention in the form of a control element which is provided for a control apparatus of an engine, especially of a motor vehicle. A program is stored on the control element which is capable of being run on a computer, especially on a microprocessor, and is suitable for executing the method according to the invention. In this case, the invention is realized by a program stored on the control element so that this control element, which is provided with the program, defines the invention in the same way as the method which the program can carry out. Especially an electric storage medium can be used as a control element, for example, a read-only-memory or a flash memory.